One application of microwave energy is to efficiently create activated species from gaseous precursors for use in plasma treatment processes, such as semiconductor etching and thin film deposition. Previous microwave plasma deposition techniques, which illustrate the state of the art and highlight both the problems encountered in increasing the energy uniformity and the advantages provided by the microwave plasma generating structure and thin film deposition apparatus of the instant invention, will be discussed in the following paragraphs.
Commonly assigned, U.S. Pat. Nos. 4,517,223 and 4,504,518 to Ovshinsky, et al., both entitled "METHOD OF MAKING AMORPHOUS SEMICONDUCTOR ALLOYS AND DEVICES USING MICROWAVE ENERGY", the disclosures of which are incorporated herein by reference, describe processes for the deposition of thin films onto small area substrate in a low pressure, microwave glow discharge plasma. As specifically noted in these Ovshinsky, et. al, operation in the disclosed low pressure regimes not only eliminates powder and polymeric formations in the plasma, but also provides the most economic mode of plasma deposition. While these patents describe the revolutionary concept of operating at low pressure and high energy density utilizing microwave energy, i.e., operating at the substantial minimum of the modified Paschen curve, the problem of uniformity of the deposition of thin films over large areas remained addressed.
Turning now to microwave applicators for large area substrate, commonly assigned U.S. Pat. No. 4,729,341 of Fournier, et al., "METHOD AND APPARATUS FOR MAKING ELECTROPHOTOGRAPHIC DEVICES", the disclosure of which is incorporated by reference, describes a low pressure microwave initiated plasma process for depositing a photoconductive semiconductor thin film on a large area cylindrical substrate using a pair of radiative waveguide applicators in a high power process. However, the principles of large area deposition described therein are limited to cylindrically shaped substrates, such as electrophotographic photoreceptors, and the teachings provided therein are not directly transferable to elongated, generally planar substrates.
While workers in the field have disclosed methods of processing thin films utilizing the high power of microwave sustained plasmas, previously microwave plasma treatments have not been altogether appropriate for large surface area and/or low pressure deposition. This is because of the non-uniformity of the plasma over an enlarged or elongated substrate due to the non-uniformity of the energy initiating the plasma. One attempt to provide greater large area uniformity was the use of a slow wave microwave structure. A problem that is inherent in slow wave structures, however is the very rapid decline of microwave coupling into the plasma as a function of distance transverse to the microwave applicator. This problem has been addressed in the prior an by various structures that vary the spacing of the slow wave structure from the substrate to be processed. In this way the energy density at the surface of the substrate can be kept constant along the direction of movement of the substrate. For example, U.S. Pat. No. 3,814,983 to Weissfloch, et al. for "APPARATUS AND METHOD FOR PLASMA GENERATION AND MATERIAL TREATMENT WITH ELECTROMAGNETIC RADIATION and U.S. Pat. No. 4,521,717 to Kieser, et al., for "APPARATUS FOR PRODUCING A MICROWAVE PLASMA FOR THE TREATMENT OF SUBSTRATE IN PARTICULAR FOR THE PLASMA POLYMERIZATION OF MONITORS THEREON", both address this problem by proposing various spatial relationships between the microwave applicator and the substrate to be processed.
More, particularly, Weissfloch, et al. discloses that in order to obtain the uniform electric field intensity necessary for a plasma of uniform power density along the full length of the slow wave waveguide structure, it is necessary to incline the waveguide structure at an angle with respect to the substrate. It should be apparent, however, that inclination of the slow wave waveguide structure with respect to the substrate, to achieve uniformity, leads to an inefficient coupling of microwave energy into the plasma.
Recognizing this deficiency, Kieser, et al. described that the conditions resulting from superposing of two energy inputs, i.e., two microwave applicators, can be further improved if the two slow wave applicators are set at an angle to each other such that the planes normal to the medians of the applicators intersect at a straight line which extends parallel to the surfaces of the substrate to be treated and at fight angles to the direction of travel of the substrate. Moreover, Kieser, et al. recommended that in order to avoid destructive interference of the wave field patterns of the two applicators, the applicators should be displaced from each other transversely of the direction of travel of the substrate by a distance equal to half of the space between the cross-bars of the waveguide. In this way the microwave field pattern is substantially suppressed.
The problem of plasma uniformity and more particularly, energy uniformity was treated by J. Asmussen and his co-workers, for example in T. Roppel, et al. "LOW TEMPERATURE OXIDATION OF SILICON USING A MICROWAVE PLASMA DISC SOURCE", J. Vac. Sci. Tech, B-4 (January-February 1986) pp. 295-298 and M. Dahimene and J. Asmussen "THE PERFORMANCE OF MICROWAVE ION SOURCE IMMERSED IN A MULTICUSP STATIC MAGNETIC FIELD" J. Vac. Sci. Tech. B-4 (January-February 1986) pp. 126-130. In these, as well as other papers, Asmussen and his co-workers described a microwave reactor which they refer to as a microwave plasma disc source ("MPDS"). The plasma is reported to be in the shape of a disc or tablet, with a diameter that is a function of microwave frequency. A critical advantage claimed by Asmussen and his co-workers is that the plasma disc source is scalable with frequency: that is, at the normal microwave frequency of 2.45 gigahertz, the plasma disc diameter is 10 centimeters and the plasma disc thickness is 1.5 centimeters; but that the disc diameter can be increased by reducing the microwave frequency. In this way, the plasma geometry was said to be scalable to large diameters, potentially yielding a uniform plasma density over a large surface area. However, Asmussen, et al. only described a microwave plasma disc source which is operational at 2.45 gigahertz, where the plasma confined diameter is 10 centimeters and the plasma volume is 118 cubic centimeters. This remains far from being a large surface area. In order to provide for the deposition onto large area substrates, Asmussen, et at proposed a system operational at the lower frequency of 915 megahertz, which would provide a plasma diameter of approximately 40 centimeters with a plasma volume of 2000 cubic centimeters. Furthermore, the deposited material quality and deposition rate is dependent on excitation frequency. The modulation of frequency to increase plasma dimensions compromises material quality and film deposition rate.
Workers at Hitachi have described, for example in U.S. Pat. No. 4,481,229 to Suzuki., et al., the use of electron cyclotron resonance (ECR) to obtain a high power plasma having a relatively high degree of uniformity over a limited surface area. However, the Hitachi patent does not teach, nor even suggest a method by which uniform large area plasmas may be achieved. Moreover, the use of ECR imposes the added requirement of highly uniform magnetic field structures in the microwave apparatus, and may be restricted in operation to only those very low pressure regimes where electron collision times are long enough to allow the ECR condition to be achieved.
U.S. Pat. Nos. 4,517,223 and 4,729,341 referred to above, describe the necessity of using very low pressures in very high microwave power density plasmas in order to obtain high deposition rates and/or high gas utilization. However, the relationship between high deposition rates, high gas utilization, high power density, and low pressure further limits the utility of slow wave structures and electron-cyclotron resonance methods. The limitations of the slow wave structure and of the electron-cyclotron resonance methods were obviated by the method and apparatus disclosed in commonly assigned U.S. Pat. No. 4,893,584, to Doehler et al., "LARGE AREA MICROWAVE PLASMA APPARATUS", the disclosure of which is hereby incorporated by reference.
However, the apparatus of the '584 patent, as well as the apparatus of the prior art, each suffer from a specific inherent design problem. That is, the prior art apparatus, due to their specific configuration, allow coating of the microwave radiating applicator isolating means (i.e. microwave window, protective cylinder, etc.) by the deposition thereonto of material intended for the substrate web. In a continuous roll to roll process, this coating of the isolating means can cause reduced microwave efficiency at the intended deposition region, overheating of the isolating means and larger mounts of "downtime" due to the necessity of cleaning or replacing the isolating means.
It would be commercially advantageous to create a deposition apparatus in which material could be deposited onto a continuous web of substrate material at multiple sites upon the web during a single pass through the apparatus. This would allow for higher overall web speed, and in the case of temperatures sensitive substrate materials, would allow for multiple cooling stages between deposition sites to prevent overheating and possible destruction of the web of substrate material.
One particular application for the deposition of thin film coatings onto an elongated relatively wide web of substrate material is for the food packaging industry. More particularly, there has recently arisen a desire to deposit thin film oxygen and water vapor impermeable coatings atop an elongated polymeric web to be employed for packaging and giving extended shelf life to perishable foodstuffs. In order to accomplish this objective, researchers have previously developed thin film SiCO coatings and have even suggested the significance of controlling the hydrogen content of that thin film.
More particularly, the importance of controlling the hydrogen content of prior art films for the purpose of, inter alia, depositing oxygen impermeable films has been discussed in commonly assigned U.S. Pat. No. 4,737,379, the disclosure of which is incorporated hereinto by reference. As was noted therein, plasma deposited amorphous silicon hydrogen alloys, as well as alloys of silicon and hydrogen with one or more of carbon, oxygen and nitrogen suffered from various shortcomings. The hydrogen content was strongly dependent upon the substrate temperature, that content decreasing at high temperatures and increasing at low temperatures. The deleterious effect of hydrogenation on film properties, such as oxygen and water vapor permeability, is a direct consequence of hydrogen's role as a chemical bond terminator. As such, hydrogen can disrupt the connectivity of the chemical bond network of the deposited film, thereby reducing its average atomic coordination number. The solution preferred by said '379 patent was to eliminate the presence of hydrogen for the feedstock gases. This was at least partially due to the fact that thermally sensitive substrates, such as plastics, could not be heated sufficiently to eliminate hydrogen bonding in the deposited thin films. This inability to drive off hydrogen produced thin films characterized by greatly deteriorated properties which limited the utility of said films. However, the recipes set forth in the '379 patent fail to provide a film which exhibits the type of oxygen and water vapor permeation characteristics demanded by the food packaging industry.
Therefore, there is a need in the art for a continuous, roll to roll deposition apparatus which substantially eliminates coating of the microwave radiating applicator isolating means by deposition material and allows for multiple simultaneous depositions upon a low temperature web of substrate material in a single pass through the apparatus. There also exists a need for the deposition of thin film oxygen and water vapor impermeable flexible coatings atop a low temperature web of substrate material. These and other needs are met by the microwave apparatus and deposition method described hereinbelow.